In brief if I am mixing equal volumes of RO water and tap water can I expect the resultant pH to remain the same as the total volume of water increases?

So; if a mix of 15 litres of RO water and 15 litres of tap water produces an overall pH of 7.0 could I expect the same outcome if I mix 18 litres of RO and 18 Litres of tap water under the same conditions.

I ask because I am finding that in the example described above I end up with a pH of 7.2 when I mix 18 litres but 7.0 when I mix 15 litres. Is there a 'tipping point' at which the harder tap water resists the effect of the demineralised RO water?

I am nowhere near an expert but you might want to look up the "titration curves" of pH.

It gets into two solutions of the same concentration, you would have to use twice the volume of acid to reach the equivalence point. And adding a high pH to a lower pH causes the liquid to form a buffer as it does its Ion exchange.

What happens when you mix differant pH's at the atomic level is pretty complex stuff. That I do not fully understand.

My guess would be that the larger volumes of water mixed 1:1 create a larger buffer which takes the water longer to exchange ions. Which would account for the pH differance after 90 minutes.

Have you noticed that the pH comes back down if you let the larger amount of water sit longer? Maybe let it sit overnight and see if the pH comes back down._________________150 Gallon (567.811768 liters) community tank, 50 Gallon African Tank, 7 reef tanks, 6 FOLR, 28 freshwater tanks